In the scanning of a television cathode ray tube, and in order to reduce visible flicker, scanning is employed in which horizontal scanning lines are scanned at intervals of one or more lines. In general a [2:1] interlaced scanning is employed, and has been widely adopted. In the [2:1] interlaced scanning system, coarse pictures (fields) formed by a single vertical scan are overlapped with each other to form a single picture (frame). In the NTSC system, for example, the field repetition rate is 60 per second, the frame repetition rate is 30 per second, and one frame incorporates 525 horizontal scanning lines. In addition, odd numbered fields and even numbered fields are offset from each other at a starting point of the horizontal scanning by one-half of the horizontal scanning duration (H), i.e., 0.5H.
When a picture signal is recorded on a magnetic tape, or on a magnetic disc, or on other recording media, it is general to assign a signal of one field to none track, or, to assign a signal of one frame to one track. In addition, in the 1 field/1 track recording method, there is a so-called 1 frame/2 track recording method in which the odd and even fields are successively recorded, and, a field recording method in which only one of the odd and even fields is successively recorded.
In the reproduction of such field recordings, a so-called field/frame conversion system has been widely utilized iin which the same track is scanned twice to form a frame signal from a single kind of field signal. The aim principally is an improvement in recording density, which, in turn, enables an extended recording and an increase in the available number of still frames.
In case where conversion is made from a field signal to a frame signal, however, if the same field signal is merely repeated twice, it is not possible to realize interlaced scanning. The reason for this is that it is necessary for the scanning that the odd and even fields to be offset by 0.5H in time relation between the vertical synchronizing signal and the horizontal synchronizing signal. If the same field signal is merely repeated, the offset in time of 0.5H does not occur.